The present invention relates to an extension drill rod for percussion drilling machines, more specifically a drill rod consisting of a rod or a sleeve that is provided with a threaded portion for engagement with another correspondingly threaded drill rod.
Various types of threaded connections for percussion drilling machines are previously known in which flank angles and pitch angles have been varied in order to optimize the threaded coupling with regard to those percussive forces subjected to the threaded connections. It is still a problem, however, that excessive rapid wear occurs because of those increased percussive forces that are used with modern drilling machines. The main reasons for rapid wear of such threaded connections are pitting damages due to local heating and formation of friction martensite.
In accordance with the present invention a new solution to this problem is proposed according to which the threaded portion of the drill rod is provided with a wear lining consisting of a layer of a material softer than that of the drill rod, usually of steel, whereby the layer is metallurgically bonded to the threaded portion of the drill rod. One suitable wear lining material is a commercially available bronze material such as copper bronze or aluminum bronze.